The present invention relates to a thermal head, and more particularly, to a substrate structure of a thermal head.
Thermal heads are conventionally used in recording devices such as facsimiles, printers, and word processors. In general, the thermal heads are fabricated as follows. First, a heat insulating glaze layer, composed mainly of silicon oxide, barium oxide, calcium oxide, and boron oxide, is formed on a high electrical resistance substrate, made of aluminum oxide, as an alkali-free ceramic material of at least 90% purity. Then, heating resistors, made of Ta-SiO.sub.2, Ta.sub.2 N, NiCr, etc., are formed in a specific pattern on the heat insulating layer. Further, conductor patterns of aluminum or gold, for use as individual electrodes and a common electrode, are formed on the heating resistors. Also, the resistors are formed at least with an oxidation protective layer made of SiO.sub.2, and a abrasion protective layer made of Ta.sub.2 O.sub.5. The oxidation protective layer serves to prevent oxidation of the heating resistors, while the abrasion protective layer prevents the head from being worn away by contact with thermal paper.
As is generally known, when a pulsating voltage is applied between the individual electrodes and the common electrode, a current is supplied to the heating resistors between the electrodes. As a result, the resistors generate Joule heat, whereby the heat-sensitive paper pressed onto the abrasion protective layer is printed. The generated Joule heat diffuses not only to the paper side, but also to the substrate side. The heat insulating layer serves to control the conduction and accumulation of the heat. Thus, the heat applied to the heat-sensitive paper can be controlled properly for satisfactory printing.
The high electrical resistance substrates of the thermal heads, as described above, are made of ceramic material, and most of them are formed of aluminum oxide with a purity of 90% or more. Steps of manufacturing one such substrate include processes of material treatment, such as removal of alkaline-metal ions, a high-temperature sintering process, and finishing processes, including a polishing process. Thus, the manufacture of the substrate requires complicated processes, and therefore entails high costs.
The ceramic material contains alkaline-metal ions. If the thermal heads are made of this material, the alkaline-metal ions may possibly precipitate in the heating resistors, threby deteriorating the characteristics of the heating resistors. In order to prevent the alkaline-metal ion separation, the alkaline-metal ions must be previously removed from the material or ceramic powder, to be reduced to a content level below a reference value, e.g., 0.05% or less, by weight, for Na.sub.2 O.
The prior art substrate is obtained by compressing ceramic powder into a predetermined shape, and the sintering the resulting green sheet at a temperature of about 2,000.degree. C. to 2,300.degree. C. Immediately after the sintering, the substrate is subject to bending and/or warping. If it is formed directly into a thermal head, therefore, the head will not be able to have uniform intimate contact with the heat-sensitive paper. Accordingly, the surface of the ceramic substrate is polished in the polishing process to remove its bending and/or warping.
Moreover, the conventional thermal heads of the aformentioned construction can be miniaturized only to a certain extent. Thus, there is a demand for the development of thermal heads of a smaller size and simpler construction.